In the technology of lithographic printing, a printer will typically produce a series of metal based lithographic printing plates which will be used to reproduce an original many thousands of times. However, metal based presensitized printing plates are expensive and errors in the image can cause considerable unnecessary expense. It has therefore become customary and desirable to produce an inexpensive color proof to assist a printer in judging the quality of, and correcting a set of photographic color separation films prior to starting the expensive lithographic printing process.
Color proofing films are used to reproduce the color quality that will be obtained during the actual printing process. The proof must be a consistent duplicate of the desired half tone or line image. Visual examination of a color proof should reveal the color rendition to be expected and any flaws on the separations which might need to be altered before making the printing plates. Color proofing sheets for multicolored printing can be made by using a printing press or proof press. This requires taking all the steps necessary for actual multicolor printing. Such a conventional method of color proofing has itself been costly and time consuming. Alternate color proofing methods have therefore been developed to simulate the quality of press proofs. There are several types of photoimaging color proofing methods. These include, namely, the overlay type and the single sheet type.
In the overlay type of color proofing, an independent transparent plastic support is used for producing an image of each color separation film. A number of such supports carrying colored images are then superimposed upon each other and held on a white sheet to produce a color proof. The overlay type of color proofing method has the disadvantage that the superimposed plastic supports tend to darken the color proof, and, as a result, the impression of the color proof thus prepared becomes vastly different from copies actually obtained with a conventional printing press. Its primary advantage is that it is quick and can serve as a progressive proof by combining any two or more colors in register.
In the single sheet type of color proofing method, a color proofing sheet is prepared by successively producing images of different colors from different color separation films onto a single receptor sheet. This is done by utilizing a single opaque support and by applying toners, photosensitive solutions or coatings of photosensitive materials of corresponding colors on the opaque support in succession. An advantage of the single sheet type of color proof is that the color saturation is not influenced by superimposed plastic supports. This method more closely resembles the actual printing process and eliminates the color distortion inherent in the overlay system.
The present invention relates to negative-working and positive-working proofing sheets of the single sheet color proofing class. Films of this type are generally composed of a sheet substrate and at least a colored photosensitive layer on the sheet. An adhesive layer may be disposed on the photosensitive layer.
An example of a negative-working, single sheet approach is described in U.S. Pat. No. 3,671,236. The presensitized element comprises a substrate having a release surface, a pigmented photosensitive layer in contact with the release surface, a pressure sensitive adhesive layer, and a barrier layer between the photosensitive layer and the adhesive layer. The primary function of the barrier layer is to prevent pigment staining on the adhesive during development. The element is used by laminating it to a receiver via the adhesive layer. The substrate is removed, the photosensitive layer is exposed, and the material is processed in a solvent medium. These steps are generally repeated three times to produce a multicolored proof.
An example of a positive-working, single sheet approach is given in U.S. Pat. No. 4,260,673. The presensitized element comprises a substrate having a release surface, a pigmented photosensitive layer in contact with the release surface, a heat sensitive adhesive layer, and a binder layer between the pigmented layer and the adhesive layer. The binder layer comprises a photosensitive compound and prevents pigment staining of the adhesive. The element is processed like the negative-working element in U.S. Pat. No. 3,671,236.
Examples of negative-working and positive-working, single sheet approaches without preventative layers between the photosensitive and adhesive layers are described in U.S. Pat. Nos. 4,650,738 and 4,659,642, respectively. The presensitized elements comprise a sheet having a release surface, a colored photosensitive layer in contact with the release surface, and an adhesive layer directly adhered to said colored layer.
The one problem with single sheet proofing systems is that the image is vulnerable to damage from its environment, i.e., from fingerprints, scratches, dust and the like. To protect the surface of the single sheet proof, it is common to laminate to such surface a thin, self supporting, transparent film of polyester or the like, or a layer of photopolymer which may be polymerized by overall exposure to actinic radiation to form a tough, protective membrane. This postexposure requires an additional processing step. These prior practices, however, generally result in the formation of a thick, glossy surface on the print which in some circumstances detracts from the desired appearance of the proof. For example, a transparent, pressure sensitive cellophane tape can be applied over the final image. However, in many instances, it is preferred that the surface of a proof exhibit a matte texture to closely resemble actual printing on stock which is not glossy.
Various means have been attempted to reduce the glossy appearance of single sheet proofs, yet none has proven sufficiently simple and effective. The application to the surface of the print of powdered pigments, pigmented sprays or lacquers, or aqueous dispersions of pigments, all as mentioned, for example in U.S. Pat. No. 4,286,046, poses problems of expense, cleanliness, and health while failing to provide a matte of acceptable uniformity, texture, and appearance.
One solution to this problem has been proposed by U.S. Pat. No. 4,719,169. In this disclosure, a thermoplastic adhesive layer and an antiblocking layer are borne on one or two temporary supports and are then laminated to the proof. The temporary supports are then stripped off. The proof is itself thereby provided with either a smooth or matte finish depending upon the surface topography of the temporary support from which the aforementioned layers were removed. The two layers are somewhat disadvantageous for some applications. The present invention seeks to improve on this patent by employing a single protective layer which has both thermoplastic and antiblocking properties. Thus only a single layer is applied to the image. This allows a covering which is thinner and reduces dot gain.